Other medications used to treat RA may also affect sperm count like leflunomide & azathioprine. As soon as you're considering starting a family, have your husband see his rheumatologist. Some medicines need a months-long "washout" period before trying to conceive. Ask if Plaquenil can be used instead of MTX (although side effects of Plaquenil are not that predictable, your doctor may need to weigh the benefits against the possible risks)
I was hoping I could provide a link to a forum thread specifically about male patients taking MTX and are trying to have a child but I couldn't find anything which have experiences from other users. I found this instead and hopefully, you find the reply helpful:
Arthritis Forum - methotrexate and pregnancy